X-ray vidicon having a target and window assembly with improved thermal conductivity



Jan. 24, w. JORDAN X-RAY VIDICON HAVING A TARGET AND WINDOW ASSEMBLY WITH IMPROVED THERMAL CONDUCTIVITY Filed Sept. 16, 1965 HVVENTOR HAROLD 0. W JORDAN BY II AGE T United States Patent O 3,300,669 X-RAY VIDICON HAVING A TARGET AND WIN- DOW ASSEMBLY WKTH IMPROVED THERMAL CONDUCTIVITY Harold O. W. Jordan, Stamford, Conn., assignor to The Machlett Laboratories, Incorporated, Springdale, Conn, a corporation of Connecticut Filed Sept. 16, 1963, Ser. No. 309,043 4 Claims. (Cl. 31365) This invention relates to cathode ray tubes and particularly to television pickup tubes of the vidicon type which receive X-ray images and convert them to output electrical signals.

X-ray sensitive vidicon tubes embodying the present invention have improved overall operating efliciency achieved by a novel target structure which, by providing better thermal conductivity between elements of the target, reduces damage caused by high temperatures during processing and subsequent operation of the tubes. Further improvement is achieved by a novel photoconductor of selenium doped with controlled amounts of arsenic, and of a thickness of about 20 microns, which permits a tube to be operated at higher target potentials such as above 30 volts with resultant substantial gain in sensitivity. Another advantage of the described photoconductor is the inhibition of surface recrystallization which occurred in many known types of photoconductors such as those formed of pure selenium.

In vidicon tubes it is desirable to employ a window which has minimum absorption of the radiation which carries the input information. In X-ray vidicons the windown is advantageously formed of beryllium which has low X-ray absorption in comparison with other metals or glass. However, beryllium cannot be easily provided with the highly polished surfaces necessary for proper support of a photoconductor such as selenium, which is commonly used for this purpose. Further, beryllium and selenium have been found to be incompatible. These factors required that X-ray vidicons be provided with thin glass layers on the inner surfaces of the beryllium windows to support the photoconductive material and isolate it from the beryllium.

However, in devices of this type it has been found difficult to provide good thermal contact between the beryllium and glass surfaces. This was caused by one or more of several things such as foreign deposits falling between the surfaces during soldering operations or unevenness in the beryllium surface. Such poor thermal contact, particularly in the central portion of the target, resulted in damage to the photoconductor on the glass disc when the target became subjected to high temperatures during tube processing or subsequent operation. Normal methods of cooling from sources external to the tube could not overcome this problem.

It is, accordingly, an objective of this invention to overcome the above problem by providing a device wherein the beryllium window has a slightly convex inner surface and the glass disc is mounted against the convex surface with its marginal areas physically urged toward the window so that the inherent flexibility of the glass permits contact throughout substantially the entire surface,

A further problem with known X-ray vidicon tubes is that they cannot be operated at relatively high target potentials such as from 30 to 70 volts, for example. Selenium is commonly used as the photoconductive material and is deposited upon an irridized surface or backplate on the glass disc to which a potential of less than about 30 volts is applied during operation of the tube. The selenium charges toward the potential of the backplate during the field period and is discharged down to cathode potential by the scanning electron beam. How- 3,300,669 Patented Jan. 24, 1967 ever, with photoconductor thickness above a few microns, the potential of secondary emission first crossover is reached by the photoconductor with backplate potential of about 30 volts. Therefore, to insure normal operation in the cathode stabilized mode the backplate potential must be kept somewhat below 30 volts. If the photoconductor is permitted to exceed the first cross-over potential, it will be stabilized to the potential of the field mesh which is normally operated between 300 and 1000 volts, thus producing across the target a voltage equal to the difference between the mesh and target potentials. The result is a reversal of signal polarity, and possible damage to the photoconductive surface due to high velocity bombardment. However, operation below the first cross-over potential has the disadvantage of limiting sensitivity which is a function of target voltage.

Accordingly, it is another objective of this invention to provide means whereby the tube may be operated at target potentials above 30 volts to increase sensitivity without fear of reversal into the high velocity mode. This is achieved by providing a photoconductor of sele nium to which is added from about 2% to 10% of arsenic. With this photoconductor the potential of first crossover is shifted to beyond the desired operating target voltage range of up to 70 volts.

Another objective of this invention is the provision of a novel method of forming the photoconductor composition and depositing it upon the supporting structure of the target, whereby the arsenic is uniformly distributed throughout the selenium and whereby the composition is uniformly deposited upon the supporting structure in controlled thicknesses.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a side elevational view partly in axial section of a tube embodying the invention; and

FIG. 2 is an enlarged sectional view of the target end portion of the tube shown in FIG. 1.

Referring more particularly to the drawings wherein like characters of reference designate like parts throughout the views, the tube embodies an elongated hollow glass envelope or bulb 10 having within one end an electron gun structure 11 which is adapted to form an elec tron beam for the purpose of scanning a target 12 in the opposite end of the envelope.

The electrodes of the electron gun 11 include the usual cathode and control electrodes, as well as the auxiliary accelerating and focusing electrodes, which are suitably connected to respective lead-in pins or terminals 13 in the well-known manner.

In the normal operation of a tube of this type a filament or heater (not shown) within the cathode-grid assembly 14 is adapted to heat the cathode to the temperature necessary to cause copious liberation of electrons from the cathode. The electrons are drawn through the electrodes and simultaneously formed into a beam which is adapted to scan the target 12. The final electrode 15 of the gun 11 is depicted as a hollow cylinder extending longitudinally of the tube from the electrode portion 16 to a point adjacent the target 12, one end of the cy-linder being Supported by the gun and its associated rigid izing structure and the other end being supported upon the inner glass walls of the envelope by resilient spacers 17. The final electrode 15 may, however, be provided in the form of a vaporized metallic coating on the inner wall of the bulb 10. The end of electrode 15 which is nearest the target 12 carries a field mesh or grid 18 in the form of a relatively fine metal mesh which is permeable to the electron beam. Grid or field mesh 18 during operation of the tube functions together with external coils and electrode 15 (not shown) to insure that the electron beam in its final approach to the surface of the target is substantially normal to that surface.

The target 12 is supported upon a faceplate which comprises a window 19 of beryllium which is transparent to X-radiation. The window 19 has a substantially planar outer surface directed toward a source of X-radiation.

The inner side of the window is adapted to support the target 12 which comprises a layer 20 of photoconductive material. Selenium is well known as a material which becomes highly conductive when subjected to X-radiation and is particularly suitable as the photoconductor in X- ray vidicons. However, selenium and beryllium are imcompatible due to the fact that beryllium enhances a detrimental recrystallization of the selenium from the amorphous to a lower resistivity crystalline form. Furthermore, beryllium surfaces cannot be conveniently prepared with the high degree of uniformity and polishing required in high resolution pickup tubes.

Therefore, an intermediate isolating layer 21 of thin glass is disposed between the photoconductor 20 and the adjacent surface of the beryllium window 10. In order to avoid excessive absorption of the X-ray image which enters the tube through window 19, the glass layer or disc 21 is made very thin, approximately five mils thickness being satisfactory.

It is important, however, that the glass be in good thermal contact with the beryllium since poor contact between these elements causes damage to the photoconductor. Considerable heat is genera-ted internally of the tube and is radiated to the faceplate during processing and subsequent operation. The tubes are usually cooled by external means, and therefore, poor contact between the glass and windows permits hot spots to be formed in the photoconductor, which hot spots cannot be efficiently cooled.

Therefore, in accordance with this invention the inner surface of the window is made slightly convex and the glass disc 21 is placed upon the convex surface and is physically urged toward the window 19 at its outer edges. Actual-1y bending of the disc occurs because of the inherent resiliency of the glass and the extreme thinness thereof. This insures good thermal contact between the glass and the window, particularly in the central area upon which the photoconductor 20 is deposited. The curvature of the inner surface of the window 19* need be slight and is shown in exaggeration in the drawings appended hereto. In actuality the center of the convex surface may be raised approximately .001 inch above the edge.

The window and target structure is mounted within a suitable supporting structure comprised of a cuplike frame or bezel 22 of metal which encircles at least the periphery of the window and has an inwardly directed annular flange 23 overlying the front or exposed surface of the window. Frame 22 is suitably soldered to the window. The whole faceplate structure is secured to the end of the envelope by a cup or ring 25 which is vacuum-sealed to the envelope at one end and is provided with an outwardly directed flange 26 at its other end. Flange 26 is adapted to be forcibly pressed against the adjacent surface of glass disc 21 to urge the disc toward the window, and is adapted to be held in place by a ring 24 of solder, as clearly shown in FIG. 2, which vacuum-seals the flange 26 to the frame 22. A vacuum-tight indium solder seal is made throughout the circumference of the device and retains the parts in assembled relation. When the solder ring 24 'has cooled and hardened, the glass disc 21 is retained in its deformed condition closely embracing the curved surface of the window 19.

The most suitable photoconductor in a tube of this type is the amorphous form of the element selenium which is usually deposited onto the window by evaporating the pure metal to the desired thickness on the glass disc 21. The photoconductor is actually deposited upon the surface of the glass disc which faces the electron gun 11, this surface being made conductive such as by being irridized by treatment with a solution of alcohol and stannic chloride (Nesa) or by being coated with a thin layer of aluminum, tin, or other metal having low X-ray absorption in a well-known manner to render the surface conductive whereby it functions as a backplatc. Thus, electrical charges are enabled to pass from the photoconductor to the backplate, indicated by numeral 27 in FIG. 2, and thence outwardly of the tube through flanges 26 and 24.

In operation of the tube, the selenium target charges toward the potential of the backplate during the initial scan or field period and is then discharged down to cathode potential by the electron beam. However, when the selenium layer is more than a few microns thick, the potential of secondary emission first crossover can be reached by the selenium with a backplate potential of approximately thirty volts. To insure normal operation in the cathode stabilized mode it is required, therefore, that the backplate potential be kept somewhat lower than thirty volts since if the target is permitted to exceed the first crossover potential, it will be stabilized to the potential of the field mesh 18 which is normally operated at somewhere between 300 and 1000 volts, thus producing across the target a voltage equal to the difference between the mesh and target potentials. This results in a reversal of signal polarity and possible damage to the photoconductive surface due to high velocity bombardment, as well as in limitation in sensitivity due to necessary operation below the relatively low first crossover potential.

In accordance with this invention, sensitivity, which is a function of target voltage, is increased by the fact that the tube may be operated at target potentials above 30 volts without fear of reversal into the high velocity mode. This is achieved by modifying the selenium photoconductor in such a way so as to shift the potential of first crossover beyond the desired operating target voltage range of up to seventy volts. The pure selenium is ground to a very fine powder and to it is added a quantity of pure arsenic, which preferably is also ground to substantially the same degree as the selenium. The arsenic is incorporated into the mixture in an amount of up to 10% of the mixture, preferably about 2%, and care is taken to insure that the arsenic is thoroughly mixed with the selenium to obtain even distribution of the arsenic throughout the mixture.

While the amount of arsenic may vary somewhat, as pointed out, any amount above 10% results in objectionable lag in the photoconductor. In fact, amounts above 5% produce noticeable lag but may be satisfactory under certain conditions.

A selected amount of the mixed photoconductor material is then deposited in an evaporator boat, the amount being dependent upon the required thickness of the photoconductive layer when evaporated to completion. The quantity may be determined by experimentation but in the case of one X-ray sensitive vidicon the evaporator system is calibrated to produce one micron thickness with 2.36 mg. of photoconductor material.

The loaded evaporator boat is mounted in a suitable vacuum system and is pumped down to approximately 5 10' mm. of mercury and then an inert gas such as argon is admitted to the system to increase pressure to approximately atmospheric pressure. The temperature of the mixture within the boat is then raised until the selenium and arsenic melt together, care being taken to keep the temperature below the boiling point of the materials to prevent loss of material by evaporation. The actual boiling point temperature of the two materials will, of course, depend upon the degree of vacuum and the particular gaseous atmosphere in which the operation is carried out. Should one material evaporate and the other not evaporate, a change in the proportions of ingredients in the mixture will occur.

The boat, thus prepared, is then mounted below a faceplate and evaporation to completion is carried out in high vacuum, such as 5 X mm. of mercury. An evaporated photoconductor having a thickness of approximately 20 microns is particularly suitable for operation in the 100 kv. range.

A iphotoconductor produced as described above possesses greater sensitivity because of the shift in first crossover potential to permit the desired increase in operating target voltage, and also possesses greater resistance to recrystallization even after many hours of tube operation.

From the [foregoing it will be apparent that a novel X- ray sensitive vidicon tube has been provided in accordance with the objectives of this invention. It will also be apparent that modifications and changes may be made without departing from the spirit of the invention as expressed in the accompanying claims.

I claim:

1. A target and Window assembly for an X-ray sensitive pickup tube comprising a metal window of material efficiently transparent to X-radiation, and having a convex surface on one side thereof,

a thin glass disc having one surface superimposed upon said convex surface of the window,

means for urging peripheral areas of the glass disc toward the window to bend the disc over the convex surface and for maintaining the disc in good thermal contact with the window,

and a layer of X-ray sensitive photoconductive material on the opposite surface of the glass disc.

2. In an X-ray sensitive pickup tube having a hollow tubular envelope and an electron gun assembly in one end of the envelope, a target and window assembly at the other end of the envelope comprising a metal window of material efiiciently transparent to X-radiation, and having a convex surface on one side thereof,

a thin glass disc having one surface superimposed upon said convex surface of the window,

means for urging peripheral areas of the glass disc toward the window to bend the disc over the convex surface and for maintaining the disc in good thermal contact with the window, said means comprising a frame encircling and sealed to the Window, and a ring having one end sealed to the end of the envelope and having a flange at its other end engaging the glass disc and fixedly connected to the frame,

and a layer of X-ray sensitive photoconductive material on the exposed surface of the glass disc.

3. A device as set forth in claim 2 wherein the frame has an annular flange which overlies the exposed surface of the window, and the flange on the ring is annular and engages the adjacent surface of the disc throughout an annular area near the edge thereof and is soldered to the frame.

4. A device as set forth in claim 2 wherein said frame and ring are electrically conductive and wherein the surface of the glass disc remote from the window is conductive and is electrically connected to said frame.

References Cited by the Examiner UNiTED STATES PATENTS 2,493,539 1/1950 Law 313-89 2,748,30'4 5/1956 Botden 313101 2,822,360 2/1958 Mayer et a1. 117-201 2,862,126 11/1958 Ploke et a1. 313-89 3,046,431 7/ 1962 Nicholson 31510 3,179,840 4/1965 Clayton.

JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

1. A TARGET AND WINDOW ASSEMBLY FOR AN X-RAY SENSITIVE PICKUP TUBE COMPRISING A METAL WINDOW OF MATERIAL EFFICIENTLY TRANSPARENT TO X-RADIATION, AND HAVING A CONVEX SURFACE ON ONE SIDE THEREOF, A THIN GLASS DISC HAVING ONE SURFACE SUPERIMPOSED UPON SAID CONVEX SURFACE OF WINDOW, MEANS FOR URGING PERIPHERAL AREAS OF THE GLASS DISC TOWARD THE WINDOW TO BEND THE DISC OVER THE CONVEX SURFACE AND FOR MAINTAINING THE DISC IN GOOD THERMAL CONTACT WITH THE WINDOW, 